Final answer:
The market structure described by the student, where there is only one seller and no close substitutes for the product, is known as a C. monopoly.
Step-by-step explanation:
A market in which there is only one seller, and there is no close substitute for the product being sold, is called a monopoly. Unlike perfect competition, where there are many firms selling identical products with no market power, a monopoly is characterized by a single firm that produces all of the output in the market and can set prices with a great deal of market power because it faces no significant competition. The definitions of monopolistic competition and oligopoly also represent types of market structures, with monopolistic competition having many firms selling similar but not identical products, and oligopoly being a market dominated by a few firms. However, the correct answer to the student's question is C. Monopoly.